Use `"\\%0"` as replacement string to escape chars in Lua rather than function

[bpj]

djot.lua/djot-writer.lua

Line 17 in a0583ef

return (s:gsub("[][\\`{}_*<>~^'\"]", function(s) return "\\" .. s end))

It is probably more efficient to use "\\%0" as the replacement rather than a function. In a replacement string %0 gets replaced with the whole match, %1 with the first capture group and so on up to %9.

[jgm]

True. Feel free to submit a PR.